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Old 17th January 2004, 02:03 PM   #1
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Default A better fan design - the search continues

I had this idea for el-cheapo cooling for class A amps with fans. I've seen fans mentioned a lot of times, but i'd like to know if someone has tried something like this... it basically a square metallic tube:

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....|------------------------|....
....|------------------------|....
....|------------------------|....
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The vertical lines would be a couple of small 12vdc PC fans running at 9v, both blowing to the same direction; the horizontal lines strips of Al running through the tube (any decent Al extrusion would do here), and i could place foam on the inside of the extreme sections to damp the fan noise even more (those pretty dots). I think i could make it real small and have it removing a LOT of heat quite silently, the thing is that dust accumulation is a concern for me.
What do you guys think? I'm running out of ideas, and i've already ran out of cash
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Old 17th January 2004, 02:39 PM   #2
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when I was in college, I did some fluid dynamics simulations for a group that designed a low-noise fan that is similar to your but not quite.

it is essentially a rotating pipe, with fins on it. Think of your diagram. and imagine the horizonal lines are a negal pipe, except that has openings along its axel (?). the pipe is driven by two electric motors on its two extreme ends. Each motor also has a conventional fan in them blowing air towards inside of the pipe.

so air would go into the pipe from the two ends, and being thrown out throw the openings on the pipe.

It has extremely low noise.
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Old 17th January 2004, 02:46 PM   #3
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There are lots of extrusion profiles designed for just that use. www.apexjr.com has some that are drilled for 8 TO-3 devices. My Hafler DH-500 has the fan inside the cabinet blowing out through the heat sink. this is more efficient than sucking the air through the heat sinks, and therefore a little quieter. at its low speeds, the airflow is inadible. There is a problem with mechanically transmitted noise. Use the largest diameter fan and heat sink arrangement you can to minimize the velocity for a given airflow. Of course, there is no free lunch - larger profiles cost more and lower velocity airflow doesn't increase the efficiency of the heatsink as much as higher for a given surface area.

If I were designing a chassis of this sort, the fan would be mounted on the heatsink, blowing air out of the cabinet. The heat sink would be mechincally isolated from the chassis. Isolators should be designed for the mass of the object to be isolated and the desired isoalaiton frequency. You could probably get satisfactory results using an isolator simply rated for the weight/number of isolators. Manufacturer's websites may help you choose the right one. Lord (US), Barry (US) and Hutchinson (France) are three manufacturers that come to mind.

I'd also damp the case panels, to prevent radiated mechanical noise. using the same stuff you would use in a car or use MDF and veneer it. Use a very soft gasket between the heatsink exit and the case. You might want to try a flared exit, using one of the available speaker port flares. Use multiple computer inlet filters to minimize the dust intake.

Good luck and let us know how it works out. I plan something similar at least for the first cut of my Aleph-Xs.
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Old 17th January 2004, 03:24 PM   #4
joensd is offline joensd  Germany
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I know it´s not a cheap solution...
Just for inspiration and entertainment.
Fischer Elektronik

Cheers
Jens
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Old 17th January 2004, 03:47 PM   #5
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Default PULL vs. PUSH

From modeling and simulation, the best cooling solution would have a large area inlet with a grating to start laminar flow, a taperring tunnel to the heat sinks and transistors, a slightly expanding tunnel to a large multi-speed fan that pulled out the heat. Heat and Mass Transfer....the largest intake air mass plus laminar air flow across each fin for the greatest contact

Putting the fan in front creates turbelant flow which reduces the amount of air that makes physical contact along each heatsink fin.

Ideally you would also want to take advantage of rising heat energy and have the air flow up. This can lead to dust problems when the amp is on the floor.
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Old 17th January 2004, 05:01 PM   #6
joensd is offline joensd  Germany
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Sorry, the link was a kind of universal link that doesn´t work for specific pages.
You´d have to click "Lüfteraggregate" to see some pro-solutions.

How much do you wanna dissipate?
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Old 17th January 2004, 05:10 PM   #7
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Hmm, I would have thought that turbulent flow would give teh best heat transfer. Lainar flow sounds like an insulating blanket of still air at the heat sink surface. No real knowledge, though.
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Old 17th January 2004, 06:25 PM   #8
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The things that worry me are noise and dust. I was considering watercooling, but for "only" 200w it beats the purpose if an air cooled system can do it just as well and (i beleive) as noiseless. Two fans at 9vdc should move more than enough air, and be reasonably quiet, even more wih damped entrances.
Dust is a necessary evil with fans - i've seen air conditioners that have a soft cloth on the air entrance, so probably it can be taken care of without restraining airflow too much. In any case, if it gets dusty a vaccum cleaner should clean it easily.

As for the mechanical vibration, i suffered them on my PC; it happens when fans get dusty, specially in the axis. I found that rubber washers (and cleaning the damn things ) do the trick just fine. Those i will be using for sure.

Now that i think of it, are fans electrically noisy? I was planning to feed them more or less directly from the supply rails of the amp.

By the way, as i recall it the laminar/turbulent airflow is a tradeoff; laminar flow gives you better air flow but turbulent flow removes heat much better IIRC. In any case, the output of a fan is quite turbulent in itself...
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Old 17th January 2004, 06:43 PM   #9
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Default Panaflo

The quietest fans seem to be the Panaflo-brand fans by Matsushita (Panasonic). When the L1A series fans (popular with PC silencers) are run at 5 volts, they are practically inaudible - you could leave them on indefinitely. Also, on the silentpcreview.com forums, there has been some talk about a 120mm Panaflo fan sold on eBay that was part of an OEM order for some company, and the fans are even quieter than the L1A 120mm for the same operating voltage, while seeming to move more air.

Some NMB (Nippon Micro Bearing, Minebea Electronics) fans are also fairly quiet when run at less than 12 volts. Dell uses them in the back of their minitower chassis.
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Old 17th January 2004, 10:44 PM   #10
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I haven't tried Panaflo, which seems highly regarded by some
PC people. However, I have some of the Papst low noise fans,
and replaced the original fan in my PC PSU with one of these.
What I did find out, however, was that the Papst motor has
1/4 the power of the original fan, and running the original on
on 6V, which should give about the same power seemed to
make that one about as silent as the Papst. Besides, in the
case of Papst I had to return severl of them because of some
clicking or ticking noise, most certainly caused by bad
manufacturing quality (and I have heard several other having
the same problem). And I always thought Papst were supposed
to be high quality!!! Lesson, runing a cheap fan on low voltage
may be just as effective and silent as a special silent fan, at a
lower price.
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